Binders for coating materials can be provided in water-dilutable form by adequately stabilizing the disperse phase of binder droplets in a continuous phase of water. This can be achieved by adding an emulsifier (external emulsification, externally emulsified resins) or by chemically modifying the binder such that it contains ionic or other hydrophilic groups in sufficient quantity (internal emulsification, self-emulsifying resins).
One known way to water-dilutable binders based on polyester resins is the polycondensation of polyols, preferably diols, with polybasic, especially dibasic, acids, where a defined excess of acid groups remains in the polyester. After at least some of the acid groups have been neutralized, the resulting polyester resin can be emulsified in water to give stable dispersions. The polybasic acids can in particular be tribasic acids having acid groups differing in reactivity, such as citric acid or sulfoisophthalic acid, for example. Polyester resins containing moieties of sulfoisophthalic acid are known from, for example, EP-B 0 649 439.
Binders comprising polyester resins of this kind are crosslinked using crosslinking agents which react with hydroxyl groups of the binders. Suitable crosslinking agents are polyfunctional isocyanates, which in unblocked form are reactive even at room temperature, or in blocked form, which are reactive only at an elevated temperature above 80° C., preferably above 100° C., or etherified or unetherified amino resins. Polyesters of sufficiently high molar mass are also physically drying; that is, after the dispersant has evaporated, the binder droplets coalesce and form a film.
From EP-B 0 649 439 aqueous dispersions of linear polyesters and vinyl resins are known which include mass fractions of from 5 to 40% of the polyester and from 95 to 60% of a vinyl polymer including a mass fraction of at least 20% of repeating units derived from styrene or a styrene derivative. These dispersions are still amenable to improvement with respect to the following performance properties: more rapid (physical) drying, particularly in the case of highly pigmented coating materials (masonry paints, road marking paints) is desirable; crosslinking with amino resins (melamine resins) requires relatively high temperatures; in clearcoat materials pops are formed even at low film thicknesses, and wrinkles are formed in the dried coating film; the resistance to solvents and aqueous solutions is in need of improvement.